Most experimental work examining the effects of retinal electrical stimulation in human patients has focused on single pulses. However, a successful implant will require continuous stimulation using high frequency pulse trains. We report here data examining stimulation using pulse trains in three patients with advanced retinitis pigmentosa implanted with a 4×4 epiretinal electrode array.

Stimuli were 200ms biphasic electrical pulse trains that varied in the number of pulses (1–45). Individual pulses had durations of 0.075ms or 0.975ms for both cathodic and anodic phases. We measured amplitude threshold using a yes-no paradigm. Additionally, we carried out brightness matching to determine the amplitude needed for two pulses of different frequencies to match in brightness.

The current needed to reach threshold decreases with the number of pulses, but the necessary total charge increases linearly. A similar loss of charge efficiency occurs supra-threshold. This efficiency decrease is more pronounced for long than short pulses, and does not depend on the inter-phase interval or on the temporal separation between pulses, over 1–40msec. These data are consistent with the idea that indirect inhibition mediated by amacrine cells may be stronger for long than for short pulses.